1. Field of the Invention
The present invention relates to a printed circuit board which electrically connects exposed conductor patterns formed on a printed circuit board to each other using a conductive joining material, such as solder.
2. Description of the Related Art
Conventionally, there has been proposed a junction structure that electrically connects exposed conductor patterns formed on a printed circuit board to each other by soldering or the like (see e.g. Japanese Laid-Open Patent Publications (Kokai) No. S61-224494 and No. H10-173335). In the following, the junction structure of a conventional general printed circuit board will be described with reference to FIGS. 4A and 4B.
FIGS. 4A and 4B are views showing an example of a first junction structure of the conventional printed circuit board. FIG. 4A is a top view of the first junction structure, and FIG. 4B is a cross-sectional view of the same.
In this junction structure, a conductor pattern on a rigid printed circuit board and a conductor pattern on a flexible printed circuit board are connected to each other by soldering.
In FIGS. 4A and 4B, reference numeral 0 designates the rigid printed circuit board, 1 a base substrate the base of the rigid printed circuit board 0, 2 a copper foil trace formed on the base substrate 1, 3 a solder resist, and 4 a double-sided flexible printed circuit board. Further, reference numeral 5 designates a base film, 6a a front-side copper foil trace, 6b a back-side copper foil trace, 7a a front-side cover film, 7b a back-side cover film, 8 an end-face through groove, and 9 a soldered portion.
Referring to FIG. 4A, in the junction structure of the conventional printed circuit board, the solder resist 3 covering the copper foil traces 2 on the rigid printed circuit board 0 is cut open to thereby form exposed copper foil traces on the rigid printed circuit board 0. Further, board-end portions of the respective front-side and back-side cover films 7a and 7b covering the respective front-side and back-side copper foil traces 6a and 6b extending on the respective opposite surfaces of the double-sided flexible printed circuit board 4 up to a board end thereof are cut open to thereby form exposed copper foil traces on the opposite sides of the double-sided flexible printed circuit board 4. Then, each exposed copper foil trace on the rigid printed circuit board 0 and the associated exposed copper foil traces on the flexible printed circuit board 4 are aligned one upon the other, as shown in FIGS. 4A and 4B, and are electrically connected to each other by soldering.
The copper foil traces 2 on the rigid printed circuit board 0 and the front-side and back-side copper foil traces 6a and 6b on the flexible printed circuit board 4 all have a straight shape with the same and fixed trace width, and are arranged in a plurality of lines at the same pitch.
However, the above-described junction structure of the conventional printed circuit board suffers from the following problems:
FIGS. 5A and 5B are views of the junction structure of the printed circuit board in which the flexible printed circuit board 4 in FIGS. 4A and 4B is in the soldered state. FIG. 5A is a top view of the flexible printed circuit board 4, and FIG. 5B is a bottom view of the same.
In the junction structure shown in FIGS. 4A and 4B, if the amount of solder flowing to a back-side copper foil trace 6b on the printed circuit board 4 during an operation for soldering the exposed copper foil trace on the rigid printed circuit board 0 and the associated ones on the flexible printed circuit board 4 is too large, a short circuit can occur between the back-side copper foil trace 6b and an adjacent one. More specifically, as shown in FIG. 5B, excess solder 10 sometimes travels along the boundary edge of an opening of the back-side cover film 7b on the flexible printed circuit board 4 and reach adjacent back-side copper foil traces 6b to form solder bridges.
Even if the amount of solder is appropriate, when the flexible printed circuit board 4 is pressed by a soldering iron during the soldering operation or when a soldering robot carries out soldering while pressing the printed circuit board 4, solder on a back-side copper foil trace can be squeezed out from the back-side copper foil trace. The squeezed-out solder sometimes travels along the boundary edge of the back-side cover film 7b to cause a short circuit with an adjacent copper foil trace 6b. 
To solve this problem, there have been proposed junction structures shown in FIGS. 6A and 6B and FIGS. 7A and 7B, for example. FIGS. 6A and 6B are views showing the shapes of copper foil traces on the flexible printed circuit board 4 in a second junction structure of a conventional printed circuit board. FIG. 6A is a top view of the second juncture structure, and FIG. 6B is a bottom view of the same. FIGS. 7A and 7B are top views of a third junction structure of a conventional printed circuit board in an unsoldered state. FIG. 7A shows a rigid printed circuit board, and FIG. 7B shows a double-sided flexible printed circuit board.
In the second junction structure shown in FIGS. 6A and 6B, each of the back-side copper foil traces 6b on the flexible printed circuit board 4, i.e. the copper foil traces on a surface of the printed circuit board 4 in facing relation to the rigid printed circuit board 0, is formed such that a boundary-side portion of each exposed back-side copper foil trace 6b toward a boundary of the back-side cover film 7b covering the trace 6b has a trace width smaller than that of a board-end portion thereof.
Further, in the third junction structure shown in FIGS. 7A and 7B, each of the exposed copper foil traces 2 on the rigid printed circuit board 0 opposed to the respective back-side copper foil traces 6b on the flexible printed circuit board 4 is formed such that a portion thereof to be covered by the printed circuit board 4 has a trace width smaller than a portion thereof opposed to the board-end portion of the printed circuit board 4.
According to the junction structures configured as shown in FIGS. 6A and 6B and FIGS. 7A and 7B, even when the amount of solder flowing in between the rigid printed circuit board 0 and the flexible printed circuit board 4 is too large, excess solder flows toward the board end of the printed circuit board 4. Therefore, solder travels along the boundary edge of the opening of the solder resist 3 from which the exposed copper foil traces are exposed, without spreading inward from the associated junctions. This prevents solder bridges from being formed between adjacent copper foil traces.
However, even in the junction structures configured as shown in FIGS. 6A and 6B and FIGS. 7A and 7B, when the amount of solder is not appropriate, or when the flexible printed circuit board 4 is pressed, solder bridging can occur, which means that neither of the junction structures is a perfect solution.